Apple is planning to introduce a new Apple TV set-top box and is negotiating with Time Warner Cable and other potential partners to add video content, according to people with knowledge of the matter. Apple is aiming to unveil the device by April and have it available for sale by the Christmas holidays, though the release date could change because the company is still in the process of securing new agreements with programming and distribution partners, said two people, who asked not to be identified because the plans are private.